In U.S. Pat. Nos. 4,307,764 and 4,822,179 folding so-called big bags, in particular for bulk materials are described. The bags generally have a square cross-section and the edges are stitched together to form seams. The seams are stitched by means of a single or double row of stitches. With these bags the seams are sealed to the required extent, so that powdered material essentially cannot escape from the bag in the region of the seams. This is particularly important if the transport and storage stresses caused by pushing, knocks, falls etc. exceed a certain level for the filled bags. Also the bag contents must be prevented from being contaminated by external influences and hazardous bag contents must be prevented from endangering the environment. This is achieved by providing the external material of the bags with a sealing coating or providing an innermost sealing sheet. In these cases, additional treatment of the bags to achieve sealing tightness of the seams results in a lengthy manufacturing process and an additional material requirement. Both of these factors increase the manufacturing costs.
In another attempt to seal such bags a textile fabric strip has been applied to the stitching or stitched into the seam fold. However, this does not achieve satisfactory sealing tightness at the seams, because the stitch holes of the seams which are produced during sewing expand when placed under a load. Bags of this kind are thus unsuitable particularly for powdered material. Moreover, the use of textile fabric strips as a sealing means, leads to increased expenditure in case of recycling of the bags which are no longer serviceable, due to the use of more than one type of material in the bag's construction.